Observation of a series of more than 50 exposures of the gentle and smooth mountain slope in the Northern Hidaka Mountains, central Hokkaido, revealed that the slope is composed of ill-sorted angular debris which is covered and/or intercalated with Holocene pumice layers. Fabric analysis of the slope deposits, including measurement of vector mean and vector magnitude of the a-axis, which is nearly parallel to the azimuth of the slope, strongly suggests that the deposit is of periglacial origin, probably transported by solifluction. The vector magnitude of the slope deposits in the study area generally is more than 65%, while that of the fluvial, debris-flow, and other non-periglacial slope deposits is below 65%.
Dating the pumice layers and Holocene andosol horizon allows us to determine the period of solifluction on the slope as follows:
Stage 1: before the fall of Ta-d pumice (9, 000y.B.P.)
Stage 2: the fall of Ta-d-short periods (not confirmed)
Stage 3: before the fall of To-c2 pumice (3, 000y.B.P.) or Ta-c1 pumice (2, 000y.B.P.)
Stage 4: after the fall of To-c2 or Ta-c1
Stage 5: after the andosol horizon (7, 000-3, 000y.B.P.)
The periods of solifluction differ with rock types and slope altitude. Granite area: most solifluction ceased in stage 1 or 2 in the area lower than about 1, 000m a.s.l., while the slope was unstable until stage 3 in the area above about 1, 000m a.s.l. Hornfels and shale (Hidaka Group) area: many localities show continuation of the solifluction until stages 3, 4, and 5, while the movement ceased in stage 1 or 2 in some localities.
In the granite area, an altitude of about 1, 000m a.s.l. seems to correspond to the lowest horizon where the solifluction occurred in stage 3.
The fact that the solifluction continued for a longer time in the hornfels and shale area than in the granite one may explain by the finer debris production in the former.